The best spot I've found lately for that specific dynamic is definitely Archive of Our Own. Their tagging system is brutal in a good way, you can really drill down. I'd start by filtering the 'One Piece' fandom for the Sanji/Nami relationship tag, then sort by kudos or bookmarks. That tends to surface the stuff the community has really latched onto. A lot of the classics from old forums like FanFiction.Net have been ported over there too.
Something that works for me is checking the bookmarks of authors who write a pairing I like. You'll find they often bookmark similar stories, sometimes ones that aren't at the top of the general sort. The vibe on AO3 skews a bit older, so you get less of the super cracky stuff and more slow-burn or character studies, which I think suits Sanji and Nami's weird dance of genuine care buried under all his performative flirting. I stumbled on this one called 'The Things We Carry' that's just them talking in the kitchen after Whole Cake Island, and it nailed that exhausted, understanding tone.
Honestly, don't sleep on Tumblr either. It's harder to search, but a lot of writers post snippets or links to their AO3 works there. If you find a blog that's heavy on that ship, their reblogs can be a treasure trove. The quality can be hit or miss, but the hits feel really personal and in-character.